Tight junctions or zona occludens assist in providing a regulated barrier between the intercellular spaces within sheets of epithelial or endothelial cells. Such tight junctions are an important aspect of the normal development of tissues such as the skin and mucosal membranes. These junctions may assist in suppressing the formation and spread of tumors. Inadequate or improperly regulated epithelial or endothelial barrier function contributes to the initiation, maintenance, and exacerbation of inflammation in tissues such as the gut, lungs, and other mucosal linings. Tight junctions separate the apical and basolateral regions of these cells' membranes, allowing the establishment of different physiological environments on the opposite sides of a cell sheet, such as the different physiological environments required for transport of materials across the intestinal epithelium.